Excitability changes in the sciatic nerve and triceps surae muscle after spinal cord injury in mice

We set out to investigate the changes in excitability of the sciatic nerve and to characterize the properties of muscle contractility after contusive injury of the mouse thoracic spinal cord.The following changes were observed in animals after SCI: 1) The sciatic nerve compound action potential was of higher amplitudes and lower threshold, with the longer strength-duration time constant and faster conduction velocity; 2) The latency of the onset of muscle contraction of the triceps surae muscle was significantly shorter in animals with SCI; 3) The muscle twitches expressed slower rising and falling slopes, which were accompanied by prolonged contraction duration in SCI animals compared to controls.These findings suggest that in peripheral nerves SCI promotes hyperexcitability, which might contribute to mechanisms of spastic syndrome.The studies of Sherrington and others showed that in chronic spinalized and decerebrated preparations reflexes were easily elicitable and responded violently to stimuli, which otherwise had no effect before injury [1,2]. Hyper-reflexia and spasticity which is velocity dependent increase in muscle tone [3], are considered as signs for corticoreticulospinal system lesions [4,5]. There is also evidence linking the development of spasticity and hyper-reflexia to changes in spinal α motor neurons excitability [6-8] spinal interneuronal hyperexcitability [9] and potentiated synaptic input with muscle stretch [10-14]. However, the exact pathophysiological mechanism which underlies muscle tone and abnormalities in reflexes is unknown. Although, there is the possibility that peripheral nerve physiology might be altered after spinal cord injury (SCI), there have been limited studies to investigate it directly. However, muscle contraction studies showed significant alteration in muscle properties after SCI [15,16] suggesting that the physiology of the peripheral axons would be altered as a result of SCI and spasticity.A recent study by Lin et al.,